column of 1-inch diameter, on the other hand, leads to Ro/r, > 100 or a coil diameter of 250 cm., a rather excessive value for practical work. Despite the simplicity of this criterion it should be kept in mind that the extra plate height, Equation 1, increases a t a spectacular rate (the fourth power) with increases in column diameter. With a given limit on this extra plate height, and other things (including particle diameter) being equal, the coil diameter
must be expanded with the square of column diameter. This criterion can be used to replace Equation 6, providing the lower limit on coil diameter has already been established for a given column. LITERATURE CITED
(1) Giddings, J. C., ANAL. CHEM.34,
1186 (1962). ( 2 ) Giddings, J. C., J . Chromatog. 3, 520 (1960).
(3) Giddings, J. C., Zbid., 16, 444 (1964). (4) Giddings, J. C., J . Gas Chromatog. 1 No. 4, 38 (1963).
J. CALVIN GIDDINQS Department of Chemistry University of Utah Salt Lake City, Utah
INVESTIGATION supported by Public Health Service Research Grant GM 10851-09 from the National Institutes of Health.
N e w Assay Value for Constant-Boiling Hydrochloric Acid SIR: The purpose of this correspondence is to call attention to a new value for the composition of constantboiling hydrochloric acid that we obtained in careful coulometric titrations on this acidimetric standard (1). The conditions needed to obtain reproducible compositions of the constant-boiling mixture were studied. With the usual equipment and procedure (2, 3) the distillation rate should be made slower than that normally recommended and should not exceed 2 ml. per minute. Two samples of constant-boiling mixture were carefully and independently prepared for coulometric analysis, a t pressures of 753.2 and 752.7 mm. Hg
(corrected), respectively. The coulometrically obtained composition values for these were, respectively, 179.952 and 179.941 air wt. grams of constantboiling distillate that contains 1 mole vacuum wt. of hydrochloric acid. These results are based on the latest value of the faraday, 96,487.0 coulombs per and are independent of equivalent (4, particular atomic weight values. Each of these results represents a concentration value that is 0.053% higher than would be given by presently accepted data (2, 3), interpolated for the particular pressure involved. It might be assumed that a similar discrepancy would be found a t other values of pressure.
LITERATURE CITED
(1) Eckfeldt, E. L., Shaffer, E. W., Jr., ANAL.CHEM.37, 1534 (1965). (2) Foulk, C. W., Hollingsworth, M., J . Am. Chem. Soe. 45, 1220 (1923). (3) Hillebrand, W. F., Lundell, G. E. F.,
Bright, H. A,, Hoffman, J. I.: “Applied Inorganic Analysis,” 2nd ed., p. 181, Wiley, New York, 1953. (4) Nat. Bur. Std. ( U . S.), Tech. News Bull. 47, 175 (1963).
EDGAR L. ECKFELDT E. W. SHAFFER, JR. Research and Development Center Leeds and Northrup Co. North Wales, Pa. NINTHConference on Analytical Chemistry in Nuclear Technology, Gatlinburg, Tenn., October 1965.
Evaluation of a Computer-Based Technique for Estimating the Limit of Detection of Chromatographic Detectors SIR: A previous paper (3) described the determination of the limit of detection of chromatographic detectors by a method that was objective and theoretically sound, but required collection of many data and a long calculation. Now, the use of automatic data recording and computing equipment, originally designed for interpreting chromatograms (2), has drastically reduced the labor involved and permitted a simple, quantitative evaluation of the method. Three flame ionization detectors have been tested, and an important implication of the theory confirmed: a detector can have markedly different limits of detection for peaks of different widths. The customary method of estimating detector noise as the maximum peak-to-peak random excursion of the base line was compared and found decidedly inferior. EXPERIMENTAL
Apparatus. Previously described data recording equipment (2) was attached t o the detector output. The data playback unit and computer ( 2 )
were also used, but an additional computer program was prepared to calculate the limit of detection. Procedure. The analysis of a small amount of reference compound (butane or hexane, depending on whether a gas or liquid sample injector was available) was recorded on magnetic tape ( 2 ) . For injectors with less than 2y0 accuracy, several analyses were made and the results averaged. The base line detector output was recorded on both magnetic tape and a strip chart for 35 minutes with the converter gain increased to 10 times its usual setting (2) to provide increased accuracy in measuring noise. A constant-voltage input to the converter was recorded to simulate a noiseless detector output. The reference compound analysis was interpreted as previously described (2) with the computer instructed to report the peak area in converter pulse units, to be consistent with the base line noise calculation. The base line tape was read with the interval setting (2, 3) a t 1.67 seconds. Each base line tape was read three times to indicate repeatability ( 2 ) . For comparison with the peak-to-peak
method, one of the strip chart base line recordings was divided into eight equal segments and the maximum peak-to peak excursion estimated in each. RESULTS AND DISCUSSION
Results. The noise estimates calculated for the “noiseless” input and one detector are shown in Table I and 11, respectively. The peak-to-peak measurements are also included in Table 11. The limits of detection for the three detectors are shown in Table 111.
Table 1.
Interval, sec. 1.67 3.33 6.67 13.33
Apparent Noise Estimates for a Noiseless Input
Noise estimate, converter pulse units 0.56, 0.77, 0.80, 0.70, 0.61, 0.55, 0.74, 0.55 0.65,0.68,0.75, 0.66 0.97, 0.88 1.14
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